BodyPix does downsample before masking OOTB, the article is doing 'medium' (50%) (though for this script we ought to move that over to the python side), it's still not 30fps though without egregiously sacrificing quality, at least on my (fairly powerful) machine unless I've missed something.

Amusingly I did some hacking on this and the current bottleneck is actually reading from the webcam which is capped at <10fps without doing anything else. Switching the capture to MJPG helps.